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Abstract

In response to Executive Order 12941 (1994), the United States
government initiated a coordinated effort to assess and mitigate the seismic
hazards of its existing owned and leased facilities. This study was contracted to
enhance that effort for low-rise reinforced masonry buildings with flexible
diaphragms. The study involved the development of systematic methodologies
for the seismic evaluation and rehabilitation of such buildings.
First, the seismic behavior of these types of buildings was characterized.
Two, half-scale low-rise reinforced masonry buildings with flexible diaphragms
were tested on the US Army Tri-axial Earthquake and Shock Simulator at the US
Army Engineer Research and Development Center, Construction Engineering
Research Laboratory. This testing provided experimental data for analytical
modeling, and provided specific technical substantiation for the generally
accepted premise that diaphragm flexibility can significantly affect the seismic
response of low-rise buildings. Following that testing, diaphragms and attached
masonry chords were removed from the shaking-table specimens and subjected to
reversed cyclic quasi-static displacements.
Observations and conclusions from physical testing were used to develop
and validate a simple tool for the analysis of these buildings. The tool was
developed in the general case and then analytically bounded for the particular case
of low-rise reinforced masonry buildings with flexible diaphragms. It was
validated in the linear elastic and nonlinear ranges using data from shaking-table
testing, finite-element modeling, and lumped-parameter modeling. Data from
previous flexible-diaphragm tests, performed by others, were reevaluated in the
context of performance-based engineering and synthesized with the analysis tool
into a coherent evaluation methodology intended to supplement the existing
methodologies of Federal Emergency Management Agency (FEMA) Documents
310 and 356.
To assess the efficacy of the proposed methodology, four existing
buildings were selected from the Ft. Lewis, Washington inventory and evaluated
for seismic deficiencies. The buildings were evaluated using procedures of
FEMA 310, US Army TI 809-05, and the proposed methodology. Results
conclusively demonstrated that the FEMA 310 methodology does not sufficiently
characterize the seismic performance of flexible diaphragm systems, and the
proposed methodology is simple, effective, and useful. Recommendations are
placed in the context of evolutionary updating of the FEMA methodologies, as
applied to specific subsets of the national building inventory.